The disclosure of Japanese Patent Application No. 2002-143902 filed on May 17, 2002 including the specification, drawings and abstract are incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a shift control apparatus for an automatic transmission and in particular relates to an art which smoothly executes shifting that requires two different engagement elements to be released and engaged (i.e., simultaneous changeover of four elements) when shifting among shift speeds.
2. Description of Related Art
An automatic transmission achieves a plurality of shift speeds by changing a gear ratio by switching a power transmission route, via speed-changing elements structured as planetary gears, using engagement and release of friction engagement elements. To achieve this, it is necessary to execute engagement and release of the friction engagement elements during shifting using, as far as possible, simple hydraulic control, while inhibiting the generation of shift shock. Accordingly, generally, an operation of the engagement elements for upshifting and downshifting is executed based on a so-called engagement element changeover operation. This operation is fundamentally based on an operation in which, in order to achieve a particular shift speed, with respect to a plurality or a single engagement element in an engaged state, another engagement element is additionally engaged, or alternatively, one of the engagement elements in the engaged state is released. However, depending on the gear train structure, there are cases when a so-called changeover operation is executed in which the engaged engagement element is released while the other engagement element is engaged.
Recently, there has been a tendency toward adopting multiple speeds in automatic transmissions in order to respond to demands for improved drivability and energy conservation through a reduction in fuel consumption. Adopting multiple speeds in automatic transmissions that is generally achieved by adding an acceleration shift speed or a deceleration shift speed to a speed change mechanism formed from multiple planetary gear sets, using an overdrive or underdrive gear, respectively. However, as disclosed in Japanese Patent Application-Laid Open No. 4-219553, an alternative approach achieves multiple speeds using a different configuration in which an input to a Ravigneaux type planetary gear set is split into a high-low dual system.
In a multiple speed gear train like that described above, there is a wide selection range of shift speeds that may correspond well with the running state of the vehicle. Accordingly, the changeover operation of the engagement elements does not simply require changeover of two elements, but instead a complicated changeover of four elements becomes necessary. An example of when this four element changeover becomes necessary is a so-called xe2x80x9cjump shift,xe2x80x9d in which shifting is performed at one time from one shift speed among multiple shifting speeds to a particular shift speed. When a multiple changeover operation of the four elements is executed, how to control the order of engagement and release of each engagement element, timing, and the like, are important issues. Depending on the type of control, smooth progression of the shifting that takes places within the transmission is impaired, which may cause a discontinuity in shifting. As a result, step-like shocks may occur during shifting, with a particularly substantial shock at the completion of shifting, or alternatively, the time required for shifting becomes longer than desired.
In view of the foregoing problems, it is an objective of the invention to provide a shift control apparatus for an automatic transmission which can inhibit the occurrence of shock during shifting while also preventing a delay between shifting, by executing shifting progression smoothly when shifting which requires engagement and release of the four engagement elements occurs.
According to an exemplary embodiment of the invention, when shifting from a first shift speed to a second shift speed, a state prior to initiation of release of a second engagement element, which acts as one of the engagement elements that are released, is controlled in accordance with a predetermined estimated rotation acceleration. As a result, an input shaft rotation acceleration during a period up to initiation of release of the second engagement element is controlled, thus making it possible for shifting to progress in accordance with an input torque at the time of shifting. Accordingly, it is possible to inhibit the generation of inertia torque caused by change of the rotation acceleration during shifting, and reduce shift shock. Moreover, it is possible to complete shifting in a shift time that accords with the input torque.
Furthermore, the exemplary embodiment of the invention may be structured such that, when shifting from the first shift speed to the second shift speed, control is executed in accordance with an input shaft rotation acceleration during release of a first engagement element that acts as the other engagement element that is released, and the input torque, prior to the release of the second engagement element. As a result, in the case that the input torque decreases during shifting, shifting progresses in line with the input shaft rotation acceleration at the time of release of the first engagement element. Thus, pleasant shifting is achieved between release of the first engagement element and release of the second engagement element.
Moreover, the exemplary embodiment of the invention may be structured such that during a final period of shifting through release of the first engagement element, the inertia torque decreases along with reduction of the input shaft rotation acceleration due to the progression of shifting, or in other words, increase in the input torque is not estimated. As a result, at the time of initiation of release of the second engagement element, a hydraulic pressure of a hydraulic pressure pump becomes high, and it is possible to inhibit both reduction or decrease of the input shaft rotation acceleration, and step-like shocks during shifting.
In addition, the exemplary embodiment of the invention may be structured such that the input shaft rotation acceleration increases when the input torque has increased. As a result, it is not necessary to change the estimated input rotation acceleration along with increase in the input torque. However, if the estimated rotation acceleration is decreased along with the input shaft rotation acceleration when the input torque decreases, the estimated rotation acceleration is decreased in a manner that includes the change of the inertia torque accompanying rotation change. As a result, the estimated rotation acceleration decreases more greatly than is required to accord with the actual input torque, and the shifting progression is delayed. However, by decreasing the estimated rotation acceleration in accordance with the input torque, the estimated rotation acceleration is decreased in a manner that corresponds well with the reduction amount of the input torque. Accordingly, it is possible to appropriately set shifting progression.
In addition, the exemplary embodiment of the invention may be structured such that the estimated rotation acceleration is determined by subtracting a decreased amount of the estimated rotation acceleration, which is determined in accordance with a difference between a maximum value of the input torque up to initiation of release of the second engagement element and the present input torque, from the maximum value of the input shaft rotation acceleration up to initiation of release of the second engagement element. As a result, it is possible to accurately determine the estimated rotation acceleration that corresponds with the input torque, and thus it is possible to set shifting progression appropriately.
Furthermore, the exemplary embodiment of the invention may be structured such that variation of the inertia torque of the rotation change due to feedback of the final period of release of the first engagement element is not estimated. As a result, it is possible to inhibit change of the input shaft rotation acceleration in the final period of release of the first engagement element.
Moreover, the exemplary embodiment of the invention may be structured such that the hydraulic pressure of the second engagement element is determined based upon the input shaft rotation acceleration, particularly in the case when the input rotation acceleration is still small with respect to the estimated rotation acceleration and the inertia torque is small, during an initial period of shifting initiated by release of the first engagement element. As a result, there is no over-reduction of the hydraulic pressure for engagement of the second engagement element, and thus it is possible to prevent the release from becoming too rapid.
Furthermore, the exemplary embodiment of the invention may be structured such that it is possible to maintain the state prior to release of the second engagement element in the same state as that immediately prior to the release. As a result, the release of the second engagement element is inhibited from occurring too early with respect to an operation of the other engagement side engagement element, and at the same time, it is possible to swiftly move to the release operation.
Furthermore, the exemplary embodiment of the invention may be structured so as to prevent the release of the second engagement element occurring too early with respect to the operation of the other engagement element to be engaged. As a result, it is possible to prevent the engine from racing during shifting.
Moreover, the exemplary embodiment of the invention may be structured so as to maintain the second engagement element in a state in which the second engagement element can respond quickly to the release operation, while at the same time preventing the release from occurring too early with respect to the operation of the other engagement element to be engaged.
In addition, the exemplary embodiment of the invention may be structured such that it is possible to set inertia torque to an appropriate state according with a degree of progression of shifting caused by the release of the first engagement element, while obtaining a shift time according with the input torque. As a result, it is possible to swiftly move to initiation of release of the second engagement element.
Furthermore, the exemplary embodiment of the invention may be structured such that engagement of a third engagement element with respect to release of the first engagement element, and engagement of a fourth engagement element with respect to the second engagement element are caused to occur in a prescribed order. Accordingly, it is possible to simplify the control required for changeover of the four engagement elements.
Moreover, the exemplary embodiment of the invention may be structured such that release of the second engagement element is initiated immediately prior to complete engagement of the third engagement element. Accordingly, it is possible to limit the time of a shifting state in which the four engagement elements are sliding to an extremely short period.
Furthermore, the exemplary embodiment of the invention may be structured such that release of the second engagement element is initiated after initiation of engagement of the third engagement element and prior to complete engagement of the third engagement element. Accordingly, a shift delay and/or degradation occurring along with initiation of engagement of the third engagement element is offset by engine racing caused by the smooth release of the second engagement element. Thus, from the point of view of overall shifting, it is possible to provide continuous smooth shifting.
Next, the exemplary embodiment of the invention may be structured such that a shift from the first shift speed to the second shift speed is achieved by moving via a shift from the first shift speed to a third shift speed achieved by operation of two of the engagement elements, to a shift from the third shift speed to the second shift speed achieved by operation of two of the engagement elements. As a result, it is no longer necessary to synchronously control three or more of the engagement elements at one time. Accordingly, it is possible to favorably improve controllability, and prevent the generation of shift shock. In addition, on the occasion of shifting, control is executed such that release of the second engagement element for shifting from the third shift speed to the second shift speed agrees or corresponds well with the state of shifting from the first shift speed to the third shift speed. Accordingly, it is possible to make the overall shifting continuous, and to improve driveability.
In addition, the exemplary embodiment of the invention may be structured such that it is possible to align the release operation of the second engagement element, that is released when moving to the second shift speed, with the state when moving from the first shift speed to the third shift speed. As a result, it is possible to make the move from the first shift speed to the third shift speed, and the third shift speed to the second shift speed continuous. Accordingly, it is possible to make the shift to the second shift speed via the third shift speed feel, from the point of view of human perception, like a single shift.
Moreover, the exemplary embodiment of the invention may be structured such that, even in the case that the input torque decreases during downshifting, control is executed to inhibit delay of the release of the second engagement element, in the same manner as downshift in which the input torque is substantially constant.